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  1. #1
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    New VS, Modulating Furnace Doing Good Job?

    Got the gas bill today for our York 80%, YP8C100, VS, modulating, NG furnace. It's part of an 18 SEER DFHP system but we have been running it on emergency heat or NG for a couple months due to some HP to furnace switch over issues discussed in another thread. We had quite a cold spell with 17 days below average temps so I am trying to sort out whether it is more efficient than the old single stage, 80%, 88,000 Btu furnace. The bill for 34 days is quite a bit higher than any of the comparable periods for the last 5 years, 190 CCF this year versus 123 CCF for last year.

    I figure that the new furnace must typically run at a much lower Btu rating. The previous furnace should have been about 80% * 88,000 or around 70,000 Btu. The new unit seems to run at a low level so I figure that it is closer to 50% (typical modulation level) * 80% * 100,000 or closer to 40,000 Btu. Sound reasonable?

    The gas bill isn't very detailed but we keep usage records from previous bills. But to accurately compare usage doesn't one need degree days? Can't say I really know what that means but seems to me our bill at the last house included this statistic. Anybody know if there are actual statistics for given time periods for our specific NW Arkansas location (72704) that can be compared?

  2. #2
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    You mean like This
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  3. #3
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    Great site, BT! That is if I know what I am looking at.

    I used the site to look at comparable periods for my current NG bill of 12/08/2009 -> 01/11/2010 (34 days) and the same 34 day period for 2008 -> 2009. Here's what I see for the "Sum:" field for Heating Degree Days (HDD) (I assume this is over the period I am looking at, right?) and my NG use (CCF):

    2008-> 2009: 927 HHD, 123 CCF NG used

    2009-> 2010: 1226 HHD, 190 CCF NG used

    HHD increase over last year is 32%
    CCF increase over last year is 54%

    Furnace efficiency doesn't look good on straight comparison of increase of HHD and NG CCF. Hmmmm. Does that mean that NG usage isn't linear when compared with HHD increase? Or????

    Another fact, I think that I raised the tstat temp 1 F this year, so that uses a bit more NG. But I wouldn't think that it would result in a 22% increase in NG usage, right?

  4. #4
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    HDDs doesn't take into account the wind.
    2 days with the same temps. But one day has a 15 MPH wind. the windy day will use more gas to maintain the same indoor temp.

    1 temp increased is about a 3% increase in fuel usage.

    Guessing your 2008-2009 average temps were 38
    And your 2009-2010 average temps were 29

    Does 67 therms more for a 9 degree colder month at a 1 higher temp sound better.
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  5. #5
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    The outdoor unit should be controlling all the dual fuel heat pump switching.

    As for higher gas usage? Where is the ductwork located?
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  6. #6
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    Quote Originally Posted by beenthere View Post
    HDDs doesn't take into account the wind.
    2 days with the same temps. But one day has a 15 MPH wind. the windy day will use more gas to maintain the same indoor temp.

    1 temp increased is about a 3% increase in fuel usage.

    Guessing your 2008-2009 average temps were 38
    And your 2009-2010 average temps were 29

    Does 67 therms more for a 9 degree colder month at a 1 higher temp sound better.
    Sounds a bit better, BT. I was rethinking my previous post and found that I have a few more pertinent facts that I should have added. Since last year a working adult has moved in. Means more HW from the NG HWH for showers and washing done separately. The other factor is a new NG gas insert in the fireplace. I estimate that we use it maybe 10-15 hours a month. So maybe I'm picking on the new furnace a bit.

    Quote Originally Posted by Freezeking2000 View Post
    The outdoor unit should be controlling all the dual fuel heat pump switching.

    As for higher gas usage? Where is the ductwork located?
    FK, I agree that the HP should be in control in the DFHP, Just having an issue right now getting it to take control. Working on it and addressing in another thread.

    The duct work is pretty good. Runs through a fairly warm garage soffit into trusses between the floors, so no attic or crawl space issues.

    The NG usage is not really a major issue, the CCF usage is for a 3500+ sqft multilevel house. I'm just trying to see if the new furnace is indeed more efficient which was definitely one of my objectives or at least hopes. It sure has proved to be more comfortable in terms of right on temp control in the main areas. Some disappointment in BR's (probably closed doors) and lower area (some of the reason for lighting up the fireplace insert) with no 2nd tstat, Guess I rationalized that with the lower air flow due to the VS blower and modulating furnace versus on/off characteristic of old single stage.

  7. #7
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    Does the additional adult. Also occasionally eat at separate times. Cooking for one uses a lot of extra everything. More food cooked at joint meal times also.

    So yes. One more person did raise gas usage some. Just can't say how much from here.
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  8. #8
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    So you went from an 8x% furnace to another 8x% furnace? x = the difference in efficiency.

    Two stage (not to be confused with modulating) is a comfort feature. Longer run time = more even temp throughout the house and less cold/hot on and off shifts.

    To be even more technical then I should be a furnace running on lower then its max stage could be measured to be less efficient then a single stage furnace. This depends on how well both are set up. The heat exchanger cant vary its size, you can only vary the blower speed, inducer speed, and gas input.

    I dont know what was going on in the other thread you mentioned but to getting the heat pump operational to your liking will save you more money then using only the furnace. Depending on your utility rates of your area of course...

  9. #9
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    Quote Originally Posted by CynicX View Post

    Two stage (not to be confused with modulating) is a comfort feature. Longer run time = more even temp throughout the house and less cold/hot on and off shifts.

    .
    He has a modulating furnace. Not 2 stage.
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  10. #10
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    Quote Originally Posted by beenthere View Post
    He has a modulating furnace. Not 2 stage.
    hahah, I thought VS = versus...derrhhhh I'm working today too! Ugh look out world!

    Well regardless. The efficiency difference between one 80% furnace and another 80% will be minimal at best. Esp. in this case with the sizing difference...

  11. #11
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    Quote Originally Posted by CynicX View Post
    hahah, I thought VS = versus...derrhhhh I'm working today too! Ugh look out world!

    Well regardless. The efficiency difference between one 80% furnace and another 80% will be minimal at best. Esp. in this case with the sizing difference...
    Even with the sizing difference, I figure that the typically modulating level puts the new furnace at a much lower effective Btu level than the single stage old unit. I just mucked up the assumptions. With the new variables, it is probably very difficult to compare the furnaces. Additional adult does do a bit of NG cooking and a fair amount additional washing and showering than previous 2 adults. The <= 60% efficiency of the NG HWH could be a significant factor.

    However, BT, the web page on HDD is very nice. But the wind variable is difficult to factor in. Probably was a bit cooler due to winds when down close to 0 F which is pretty cool for here. That and the 17 days below average during the billing cycle.

    Appreciate the input, Guys.

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